6000 mail boxes means 6000 people using your site. That very high traffic and email is a frequently checked thing. SCSI would deal with the disk useage a lot better than IDE would. A 36GB drive would be good, considering you will need up to 24GB for user data alone (assuming you dont want to oversell) plus room for the OS, swap, and the other basics. Putting those on sepearate drives from the data would help also.
If it is non-corporate client [meaning no large Office documents as attachments] you can easily put 6000 email boxes on a P3 512 mb RAM with 2 x 60 GB IDE hard disks. [if you use it exclusively for mail that is]
If it is corporate, depends on their usage.
Largely depends on the OS and the MTA rather than mere hardware config
The config I gave can send and receive 200,000 - 300,000 non-attachment mails a day!
Email me if you need more info
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